The government is bracing for a Lords vote on its controversial health reforms on Wednesday, with one minister warning that any amendment delaying the bill could have "grave implications" and kill it off.

The amendment – tabled by two crossbenchers, Lord Owen and Lord Hennessy – could gain a majority in the house and either severely delay or damage the health reforms beyond repair.

The health minister, Earl Howe, has written to peers saying this could completely derail the NHS bill.

Owen and Hennessy would like to see the third section of the legislation, which deals with ideas of competition in the NHS, sent to a select committee with the ability to call witnesses for further consideration.

If all crossbenchers opted to support the Owen-Hennessy amendment, along with all Labour peers, the government could be defeated and forced to endure the bill being funnelled through the Owen-Hennessy special committee.

Under the amendment, the special committee would report back by 19 December, increasing the prospect that the legislation would not be completed by April.

In recent votes crossbenchers have shown little hesitation in voting against the government, even though the upper House is mindful that it should not get into the habit of denying the will of the directly elected lower House.

In a letter sent to peers before the debate, Howe reminded his colleagues that legislation could not be rolled over after April. He wrote: "The potential for slippage in the timetable carries grave implications for the government's ability to achieve royal assent for the bill by the end of the session.

"The house must have proper time to examine the bill, but the proposal put forward by Lord Owen could result in delay which could well prove fatal to it. This is not a risk that I believe this house should take."

Owen and Hennessy think select committee scrutiny is necessary because the bill raises serious constitutional questions, including the role of the health secretary in overseeing the NHS and the role of a new body, Monitor, in promoting competition in the health service.

Peers are under pressure to give the bill more scrutiny. In an open letter, more than 250 child health experts warned that the NHS reforms would prompt the kind of failings in the health and social care services that led to the death of Baby P, Peter Connolly.

"Peter Connolly died because too many unco-ordinated and fragmented services, staffed by too few and inexpert staff, were involved in his care. The current proposals, designed to increase provider plurality, will amplify these deficits," the letter said. "Children and young people are especially susceptible to the heightened degree of commercialisation and marketisation ushered in by this bill.

"The inevitable fragmentation of NHS services will undermine our abilities to protect vulnerable children at risk from neglect and abuse, and compromise the safety and quality of care we provide for children with serious and life-threatening illnesses.

"Plurality and competition will disperse the already stretched service beyond being sustainable."

Tuesday was the first of a two-day debate extended from one after 100 peers said they wanted to speak.

Peers from both coalition parties are under pressure to vote for a delay or a complete halt to the bill, but their whips are clear they must accept the legislation in its current form.

Earlier in the year, Baroness Shirley Williams led a rebellion that forced the government to temporarily stop the legislation. The rebels extracted concessions. Lib Dems have been warned by their leadership it would not look gracious to call for further concessions now.

Williams told the Lords: "Those of us who take the view that there has to be a very careful look at this bill, not least the issue of the responsibility of the secretary of state, aren't saying for a moment that there is no role for the independent sector, no role for innovators. But that must be within the framework of a National Health Service as a public service."

She warned ministers they had to have public backing, saying: "We won't carry the public of England if their single greatest fear appears to be sustained, that is … to move away from the concept of an altruistic health service to one that is essentially money based."

Owen had suggested that, if Lib Dems voted through the legislation, they would not be acting in the spirit of their liberal forebear William Beveridge, whose original 1942 pamphlet, when implemented by Clement Attlee, established the NHS.

Earlier in the day, Lansley's office released the contents of a letter he sent to Hennessy which denied that the bill removes the health secretary's ultimate responsibility and accountability for the NHS. The letter suggests the government is not about to capitulate to the two crossbenchers.

Lansley wrote: "Rather than pretend that somehow the secretary of state is responsible for all clinical decision-making in the NHS, the bill recognises that expertise for such decisions must sit with those health professionals closest to patients."